The present invention relates to a sensor apparatus for transmitting electrical pulses from a signal line into and out of a vessel to measure a process variable. A single conductor surface wave transmission line (Goubau line) is adapted as a sensor for industrial process variable measurement, in particular for level measurement. Such devices are intended for use for example in the process and storage industry.
A pulse sent down a probe is affected by any change of the electrical properties of the surroundings of the probe. A material located inside the vessel for example causes a change in electrical impedance at the material surface. At least part of the pulse will thus be reflected at the surface. The level of the material inside the vessel can be determined from the time required for the pulse to propagate to the surface and back.
Other process variables can be determined. The amplitude of a reflected pulse for example is a measure of the change in impedance at the reflecting surface and can be used to determine the dielectric constant of the material. Also it is feasible to measure thickness and/or dielectric constants of layers of different materials stored in a vessel from the amplitude and the time-of-flight of the respective number of reflected pulses. This is often referred to as interface measurement.
Recent developments by the National Laboratory System now make it possible to generate fast, low power pulses, and time their return with very inexpensive circuits. See, for example, U.S. Pat. No. 5,345,471 and U.S. Pat. No. 5,361,070 assigned to The Regent of the University of California. The pulses generated by this new technology are broadband, and also are not square wave pulses. In addition, the generated pulses have a very low power level. Such pulses are at a frequency of 100 MHz or higher and have an average power level of about 1 nano Watt or lower. These factors present new problems that must be overcome to transmit the pulse down and back and to process and interpret the returned pulses.
It is of essential importance to provide a design for the sensor apparatus which ensures a high mechanical stability suitable for industrial applications while at the same time maintaining the electrical operation of a Goubau line. This includes ensuring a smooth impedance transition of the pulse from the signal line and transmission through the mounting to the probe and vice versa. Changes in electrical impedance throughout the apparatus, i.e. the signal line, the mounting area and the probe inside and outside the mounting section are to be avoided. Electrical impedance discontinuities and/or geometric discontinuities cause a partial reflection of energy of the pulse and thus reduce the signal to noise ratio. This can result in disruption, dissipation and/or excitation of modes of propagation other than the ones originally excited.
In U.S. patent application Ser. No. 08/574,818 entitled SENSOR APPARATUS FOR PROCESS MEASUREMENT filed on Dec. 19, 1995, and now U.S. Pat. No. 5,661,251, issued Aug. 26, 1997; and a related Continuation in Part Application U.S. patent application Ser. No. 08/735,736 with the same title filed on Oct. 23, 1996, now U.S. Pat. No. 5,827,985, issued Oct. 27, 1998, sensor apparati for transmitting electrical pulses from a signal line into and out of a vessel to measure a process variable are described.
A sensor apparatus is described comprising:
a mounting section configured to be coupled to the vessel
dielectric inserts located inside the mounting section,
said dielectric inserts having central apertures,
a conductive probe element mounted inside the mounting section and extending through the apertures of the dielectric inserts into the vessel,
an electrical connector
configured to couple the signal line to the probe element,
wherein an electric impedance inside the mounting section, is nearly constant and approximately equal to the electric impedance of the signal line.
The dielectric inserts are discs, cylinders or cones comprising two ring-shaped flat surfaces and are piled one on top of the other. A metal insert comprising a thread is screwed onto the dielectric inserts into the mounting section in a direction towards the vessel. It prevents a movement of the dielectric inserts in a direction away from the vessel. It is also described, that the metal insert alternately may be snapped in and held with a spring element and a retaining ring.
The metal insert includes an air filled conical cavity. A corresponding conical internally threaded metal fastener, serving as an impedance transitioning element and as an intermediate connecting stuctural element, is located within the air filled cavity. A mechanically reciprocating high frequency electrical connector is coupled to an end of the conical fastener racing away from the vessel via a high frequency contact pin. This pin is affixed within an aperture of the cone. The reciprocating joint of the contact pin and the high frequency connector permits some axial movement of the probe element. If the position of the conical fastener with respect to the insert is altered, the impedance matching essential for a smooth impedance transition from the signal line to the probe is impaired. Also a movement of the pin within the connector may change the electric properties of the connector and thus affect the quality of the signal transition.
The dielectric constants of the inserts are selected to minimize impedance transitions encountered by the electrical signal passing from the connector through the contact pin and into the probe element.
Given a predetermined impedance of the signal line, typically 50 Ohms, recent studies of the applicant have shown, that by employing a design as described above using layers of dielectric elements piled one on top of the other, it is not possible to build sensor apparati having mounting sections with small outer diameters. Since materials available for dielectric inserts only cover a limited range of values of dielectric constants, the minimal outer diameter is set by the impedance of the signal line and the dielectric constant of the dielectric insert .
Theoretically it is possible to dramatically reduce the outer diameter, if the design were to include air or gas filled cavities rather than dielectric inserts made of solid dielectric material There is a limitation to this though because the dielectric inserts not only serve the purpose of impedance matching but also of securely affixing the probe element within the mounting section. Obviously air or gas filled cavities even though they may have the optimum dielectric constant, are not suitable for physically supporting the probe element.
It is an object of the invention to provide a sensor apparatus which can be used in industrial applications, which has a low outer diameter, preferably of 1xc2xdxe2x80x3 or lower, while providing high mechanical stability and maintaining a high degree of transmission efficiency through the use of impedance and propagation mode controlling techniques.
To this end the invention comprises a sensor apparatus for transmitting electrical pulses from a signal line into and out of a vessel to measure a process variable, the sensor apparatus comprising:
a mounting section configured to be coupled to the vessel,
at least two concentric at least partially overlapping dielectric inserts stacked inside one another located inside the mounting section,
said dielectric inserts comprising central apertures,
a conductive probe element mounted inside the mounting section and extending through the apertures of the dielectric inserts into the vessel,
an electrical connector
configured to couple the signal line to the probe element,
wherein an electric impedance inside the mounting section, is nearly constant and approximately equal to the electric impedance of the signal line.
According to a refinement of the invention each dielectric insert has one end facing towards the vessel and one end facing away from the vessel, and all ends located inside the mounting section have outer and inner surfaces, said surfaces being orientated such that the thickness of the insert decreases towards said ends, and inner and outer diameter of each dielectric insert inside the mounting section are constant or change gradually in a direction parallel to a longitudinal axis of the respective dielectric insert.
According to a refinement of the invention an outer diameter of the mounting section is equal to or smaller than 1xc2xdxe2x80x3 and at least two of the dielectric inserts are made of different materials with different dielectric constants.
According to a refinement of the invention the dielectric inserts are made of thermo or thermoset plastic, elastomer, ceramic, polyetherimid (PEI), polytetrafluoroethylene (PTFE), polyphenylsulfide (PPS), or polycarbonate or are formed by air or gas filled cavities.
According to a refinement of the invention the dielectric inserts are clamped between a first and a second metallic insert, said first metallic insert being located near an end of the mounting section facing toward the vessel and said second metallic insert being located near an end of the mounting section facing away from the vessel.
According to a refinement of the invention the first metallic insert is pressed towards the second metallic insert by a first spring element and the second metallic insert is pressed towards the first metallic insert by a second spring element.
According to a refinement of the invention the electrical connector is attached to the second metallic insert and an external mechanical force will cause the dielectric and the metallic inserts and the electrical connector to perform incremental axial movements inside the mounting section as one integral unit.
According to a refinement of the invention an inner diameter of each metallic insert changes gradually in a direction parallel to a longitudinal axis of the respective metallic insert.
According to a refinement of the invention, the probe element comprises an downwardly tapered surface facing towards the vessel and engaging a downwardly tapered inner surface of an innermost dielectric insert preventing a movement of the probe element in a direction towards the vessel and an upwardly tapered surface facing away from the vessel and engaging an upwardly tapered inner surface of said innermost dielectric insert preventing a movement of the probe element in a direction away from the vessel.
According to a refinement of the invention an upwardly tapered inner surface of the mounting section located at the end of the mounting section facing towards the vessel engages an upwardly tapered outer surface of a first dielectric insert and an upwardly tapered inner surface of said first dielectric insert engages an upwardly tapered outer surface of the probe element located near the end of the mounting section facing towards the vessel.
According to a refinement of the invention, seals, in particular o-rings, are inserted near the end of the mounting section facing towards the vessel between the mounting section and the first metallic insert, and/or between a first dielectric insert and the first metallic insert, and/or between the probe element and the first dielectric insert surrounding it and wherein the seal between the mounting section and the first metallic insert forms a slidable seal.
According to a refinement of the invention the electrical connector is coupled to a contact pin element, said contact pin element being configured for affixation to the probe element.
According to a further refinement, the dielectric inserts are symmetric to a longitudinal axis of the sensor apparatus.
The invention and its advantages are explained in more detail using the figures of the drawing, in which two exemplary embodiments are shown The same reference numerals refer to the same elements throughout the figures.